It has generally been known that the shadow mask type color cathode ray tube causes rise in its temperature by impingement of electron beam to bring about thermal deformation. This thermal deformation brings about color difference which is generally called "mis-registration". On account of this, it has been desired that appropriate measures be taken for each cause of the thermal deformation.
As one of the causes of such heat deformation, there is a phenomenon called "doming". This is a phenomenon such that, when only a part of an image reproducing screen (a relatively large part having an area ranging from several fractions to several tens of fractions of the total effective screen area) becomes considerably bright in comparison with other part of the image reproducing screen during operation of the color cathode ray tube, and a substantially immobile image is maintained thereon over a lengthy period of time, a portion of the shadow mask corresponding to this bright portion on the image reproducing screen brings about local heat deformation.
In the following, detailed explanations will be given as to this doming phenomenon in reference to FIGS. 1A and 1B of the accompanying drawing. FIGS. 1A and 1B are schematic diagrams showing a relationship between an image reproduced on the image surface (or screen) of a color cathode ray tube and the doming phenomenon of the shadow mask. In FIG. 1A, a reference numeral 1 designates a panel with a fluorescent coating having been applied on its inner surface, and a numeral 2 refers to a shadow mask. By the way, a holding mechanism, electron guns, and other accessories for the shadow mask 2 are omitted from the illustration. On the other hand, in FIG. 1B showing an image reproduced in the screen of the color cathode ray tube, a reference numeral 10 indicates a considerably bright portion in comparison with other portions such as, for example, white clouds in the blue sky. If and when such condition continues a slightly longer period (five seconds or longer) in a substantially immobile state, a portion in the shadow mask 2 corresponding to the bright image 10 increases its temperature locally owing to much quantity of the electron beam impingement, whereby the shadow mask 2 deforms its shape from what it has primarily to be as shown by a dot line 21 to what it is shown by a solid line 22. On account of this, apertures 3 formed in the shadow mask 2 to permit passage of the electron beam can no longer maintain their constant positions. As the result of this, there is brought about deflection in quantity of irradiation on the dots of the fluorescent material, and the color difference is generated. In a general shadow mask, this heat deformation takes place in the direction where a portion of the shadow mask corresponding to the bright portion of the screen bulges out toward the fluorescent surface side. This phenomenon becomes rapidly conspicuous when the angle of deflection becomes large. This constitutes a serious problem with the recently developed color cathode ray tube having a wide angle of deflection of 110.degree..
The reason for such doming phenomenon to take place is that, when a part of the shadow mask 2 increases its temperature locally by impingement of the electron beam as shown in FIG. 1A and the temperature-increased portion thereof tends to expand, the surrounding part of the shadow mask 2, which has not been subjected to the electron beam impingements, does not expand. In connection with this, it has been known that, if the heated portion is circular in shape and the center part thereof represents a doming quantity in terms of a quantity W which moves perpendicularly to the surface of the shadow mask 2, the quantity W is approximately proportionate to a radius R of the spherical surface of the shadow mask, provided that the surface of the shdow mask 2 is spherical and is isotropic from the viewpoint of its mechanical strength. That is to say, it may be expressed as : W=.alpha..multidot.R.
Derivation of the above equation is described somewhat specifically in a periodical "The Journal of the Institute of Television Engineering of Japan", Vol. 31, No. 6 (1977), pages 49 to 50, for example.
It should be noted here that the proportional constant in the above equation includes a thermal expansion coefficient of the material for the shadow mask, but constants relating to a strength against deformation such as an elastic constant of the material do not come into the proportional constant in an express form. The reason for this is considered due to the fact that the thermal deformation takes place by the dynamics between the heated portion of the shadow mask and the non-heated portion thereof surrounding the heated portion. Accordingly, so far as the neighborhood of the portion in question of the shadow mask 2 is made up of the same material, if the mechanical strength of the heated portion is high, the mechanical strength of the surrounding portion is also high with the consequence that the deformation of the shadow mask by its doming which takes place by force exerted from the surrounding region will be the same.
Now, since the doming quantity is in proportion to the radius of the spherical surface of the shadow mask, if a reciprocal l/R of the radius R is called "a radius of curvature", the doming quantity would be in inverse proportion to the radius of curvature. Hence, the larger the radius of curvature is, the less can be made the doming quantity.
By the way, the shape of the shadow mask and mechanical strength of the material therefor are not isotropic in general. A typical example of this is a shadow mask for a color cathode ray tube having a stripe-patterned fluorescent surface. In such shadow mask, the apertures formed in its surface are long and thin, and are arranged in series in one direction, while they are arranged in a spaced apart relationship in the direction perpendicular to the abovementioned one direction, in which the apertures are arranged in series. Also, the shape of its surface is not necessarily spherical.
For the purpose of discussing this case, a normal line is erected at the point in question of the curved surface on the shadow mask 2 as shown in FIG. 2, which is made an axis Z, and then axes X and Y, which are perpendicular to this axis Z, and orthogonally intersect each other, are determined. In this connection, the direction of the axes X and Y is practically determined in relation to the arrangement of apertures in the shadow mask 2, as will be mentioned at a later paragraph; but, it needs not always be adhered to this rule in general.
Now, when the shadow mask 2 is cut along a plane X-Y, there appears a curved line at the cut face thereof. A radius of curvature of this curved line is taken as Kx, and a radius of curvature of a curved line to appear at a cut face, when the shadow mask is cut along a plane Y-Z in the same manner as mentioned in the preceding, is taken as Ky.
Upon completion of the abovementioned preparation, an equivalent radius of curvature, i.e., an effective radius of curvature, of the shadow mask 2 is considered. That is to say, in case the material for the shadow mask 2 or the shape of the surface thereof has anisotropy, a consideration is given as to the equivalent radius of curvature of the shadow mask, with which the doming is in inverse proportion. Then, from the magnitude of the equivalent radius of curvature, the magnitude of the doming is judged, on the basis of which a structure of the shadow mask having a small degree of the doming is to be found out.
In the first place, a consideration is given as to a case, wherein the material for the shadow mask 2 is isotropic and the radius of curvature thereof alone is taken up as a problem. If the shadow mask 2 has a spherical surface, Kx=Ky and this relationship should be considered equal, as it is, to the equivalent radius of curvature of the shadow mask. In the case of Kx.noteq.Ky, the following equational relationship should be established at least approximately in consideration of the fact that a mean radius of curvature K should exist between both Kx and Ky: ##EQU1## Hence it can be readily inferred that the above equation denotes the equivalent radius of curvature. Incidentally, the ordinary shadow mask has its convex surface toward the panel, hence the symbols Kx and Ky are the same.
In the next place, consideration is given as to a case of the mechanical strength of the material for the shadow mask 2 having anisotropy. The term "mechanical strength" characteristic has something to do with the Young's modulus of elasticity, the flexural rigidity, and so forth of the material. In this specification, however, it is only limited to a qualitative definition and is expressed by .sigma., wherein consideration is given mainly on a mechanical strength corresponding to a pressure required for the center part of the material to displace by a certain very small quantity, when a pressure is applied to both ends of the long (and thin) material having a certain definite small width and a certain definite length, which can be regarded as being substantially flat and which is held vertically at its both ends in a test machine.
Now, if the material has anisotropy, this mechanical strength characteristic may be considered in two cases of the abovementioned test material being cut thin and long in parallel with the direction X from the portion in question of the shadow mask 2, and of the test material being cut thin and long in parallel with the direction Y. The mechanical strength characteristic in both cases is expressed as .sigma.X and .sigma.y, respectively.
When .sigma.x and .sigma.y are the same (i.e., in case the mechanical strength characteristic of the material is isotropic), the magnitude of the mechanical strength has no bearing on the doming quantity, and the doming phenomenon takes place in inverse proportion to the mean radius of curvature to be expressed by the above equation (1). If it is assumed that the value of .sigma. becomes maximum or minimum in the direction of either X or Y (in an oblique direction other than X and Y, the values do not become maximum and minimum), the doming phenomenon is observed as a sort of average of the phenomenon in both X and Y directions, which is foreseeable as a matter of course. Even if .sigma.x .noteq..sigma.y, when Kx=Ky, it is nothing but strength .sigma. of the material having taken a certain value between .sigma.x and .sigma.y, whereby it can be anticipated that the doming quantity has no bearing on the mechanical strength .sigma.. On the other hand, when Kx.noteq.Ky and .sigma.x=.sigma.y, the above equation (1) establishes, as a matter of course, as an equation for giving an equivalent mean radius of curvature, so that the doming phenomenon has no bearing on the mechanical strength .sigma..
In the case of Kx.noteq.Ky and .sigma.x.noteq..sigma.y, the situation becomes different. That is to say, the degree of contribution of Kx or Ky to the mean radius of curvature increases with increased mechanical strength .sigma.. Considering that the relationships which have so far been described, inclusive of the equation (1), should be expressed naturally, the following equation should be arrived at as the equivalent radius of curvature. ##EQU2##
The substantial accuracy of this relationship can be verified by a simple experiment. This is also apparent from various literatures which have already been published, such as, for example, "A Newly Designed Shadow Mask for Color Picture Tube" (SID Japan Display '84 Report No. 1.4).
The shadow mask of the color cathode ray tube having the stripe-patterned fluorescent surface, which is taken up here as an example, is usually in the shape as shown in FIG. 3. The shadow mask 2 has a multitude of rectangular apertures 3 which are arranged in substantially parallel rows. If the longitudinal direction of the rectangular apertures 3 is taken in the Y direction, the apertures 3 are disposed in contiguity one after the other in the Y direction through a thin bridge 23, while they are disposed in the X direction at a spaced interval through a belt-shaped shadowing part 24 which is a non-permeable portion of the electron beam and has a width twice or more as broad as that of the aperture 3. The bridge 23 is a member provided for maintaining the shape of the shadow mask 2 per se, which is an unnecessary portion from the standpoint of the operational principle of the color selection and is said to be better if it is as thin as possible from the viewpoint of brightness characteristic of the color cathode ray tube. On account of this portion being thin, the afore-mentioned mechanical strength characteristic .sigma.X of the shadow mask 2 is considerably smaller than y. Accordingly, the shadow mask used in a television receiving set and is generally considered to have its mechanical strength relationship of .sigma.x : .sigma.y=3 : 7, or so. By the way, the strength .sigma. other than in the directions X and Y takes a mean value between .sigma.x and .sigma.y.
On the other hand, the radii of curvature Kx and Ky at each portion of the shadow mask 2 are determined by the shape of the inner surface of the panel 1, the space intervals among the apertures 3, and the positional relationship among the electron guns (not shown in the drawing), which are generally composed of plural numbers. With the recently developed color cathode ray tube, both Kx and Ky are not constant over the entire surface of the shadow mask, but are in a relationship of Kx.noteq.Ky.
Now, in the shadow mask 2 shown in FIG. 3, since .sigma.x&lt;.sigma.y as already mentioned in the foregoing, if Kx&lt;Ky, a larger value of Ky would remarkably affect the equivalent radius of curvature K based on the above equation (2) with the consequence that the value of K can be made relatively large. However, if Kx&gt;Ky, the especially large value of Kx does not affect so much on the value of K, because .sigma.x is relatively small in comparison with .sigma.y, with the consequence that the value of K is limited to a small value by a relatively small value of Ky.
Although it may be preferable, if the value of Ky can always be made greater than Kx, as there are very many constants relevant to deteremination of the geometrical radius of curvature, it is usual that the value cannot be determined freely. In particular, with the recently developed color cathode ray tube, the major axis direction of the panel in a usually rectangular shape is roughly coincident with the direction X in FIG. 3. However, since the radius of curvature of the inner surface of the panel is in most cases determined by various external factors so that the portion of the panel, on which the fluorescent surface is to be provided, may appear as flat as possible, attempts have been made as to remarkably reducing the radius of curvature of the panel in its Y direction at the end of the X direction. On account of this, the radius of curvature Ky of the shadow mask in the Y direction, which is to be determined with a certain relationship being maintained with the inner surface of the panel, should in most cases be forcibly made small (even in this case, Kx is possibly set at a fairly large value).
Accordingly, the conventional shadow mask is forced to have the relationship of Kx&gt;Ky and .sigma.x&lt;.sigma.y, with the consequence that the value of the mean radius of curvature K becomes small. On account of this, the doming quantity increases to disadvantageously bring about the color difference.
Furthermore, in recent years, it has become a general practice to make the inner surface of the panel 1 to have a relatively complicated curviform so that the external appearance of the panel 1 of the color image receiving tube may appear as flat as possible. As the result of this, the curved surface of the shadow mask 2 to be determined in relation to the shape of the inner surface of the panel 1 becomes also complicated, and the shadow mask having the radius of curvature thereof having the relationship of Kx&lt;Ky at least in one part thereof comes to be used. As described in the foregoing, the relationship of .sigma.x&lt;.sigma.y at such portion in the shadow mask effectively increases the equivalent radius of curvature and reduces the doming phenomenon. Therefor, the shadow mask 2 having the rectangular apertures 3, as shown in FIG. 3, has already satisfied the relationship of .sigma.x&lt;.sigma.y. If it is further possible to make this difference larger, the doming phenomenon can be much more reduced by more effective utilization of the relationship of Kx&lt;Ky.